Examples of machines for dewatering suspensions which contain substances whose particles are so fine that it is difficult to dewater them, such as sewage sludge, generally include various kinds of dehydrators such as vacuum dehydrators, centrifugal dehydrators, filter presses and belt presses. However, the dewatering performances of vacuum dehydrators, centrifugal dehydrators and the like are limited to a water content of about 80%, dehydrators for suspensions which are difficult to dewater there are now generally used filter presses and belt presses.
Belt presses are arranged such that about ten or more rolls are combined to stretch two filter, cloths having a mesh size of about 0.5 mm or less in such a manner that the filter cloths are able to travel, and a suspension which is passed between the filter cloths is pressed so as to be dewatered by means of a tensile force of the belts. However, such belt presses suffer from the problem that the filter cloths become clogged, thus involving troublesome maintenance.
In dehydration of suspensions which are difficult to dewater, it is general practice for improving the dewatering performance to add a polymeric flocculating agent or the like to a suspension, thereby flocculating particles therein and thus effecting dehydration. But still the water content after the dehydration is said to be generally limited to 70%.
In countries, particularly those which have limited land spaces, disposal of various kinds of sludge now becomes a problem, and there is a question as to how the use of reclaimed lands can be ensured in several years' time. For this reason, the disposal of sewage sludge is gradually being carried out more frequently by the volume reduction treatment in which the volume of sewage sludge is reduced by thermal disposal.
In the case of thermal disposal of sludge, the water content at which self-burning can take place, that is, at which sludge itself can burn without the need of an assisting fuel oil such as heavy oil, is from 65% to 70% for sewage sludge, although such water content depends on the kinds of sludge. However, since existing dehydrators do not have the capacity for dewatering sewage sludge to a water content at which it is able to self-burn, the fact is that sludge is subjected to thermal disposal with an assist from an oil such as heavy oil (about 100 l per ton of sludge), which means that disposal of sludge involves high costs. Because of the desire to conserve energy, those skilled in the art set the goal at incinerating sludge without using any oil, and various kinds of sludge incinerating systems have already been developed for the purpose of saving and creating energy. Principal examples of such systems include those wherein pulverized coal is added to sludge in order to obtain a higher heating value, those which utilize waste tires, and those which involve thickening by evaporation, drying, high-concentration dehydration, etc., and the sludge drying system has gradually become the most commonly used of the sludge incinerating systems. The sludge drying system is arranged such that a waste heat boiler is installed to dry sludge by means of steam so that the sludge becomes able to self-burn, but the system has the disadvantage that the installation cost is gigantic. Therefore, the appearance of a dehydrator which has a simple system arrangement and is still able to perform highly efficient dehydration is eagerly awaited, and dehydrators obtained by variously improving the above-described belt press which is the simplest system have already appeared.
One example of the above-described conventional dewatering apparatus is schematically shown in FIG. 18. This dewatering apparatus is arranged such that filter members 55 which are provided with a multiplicity of small bores for defining filtrate passages are disposed in opposing relation to each other, and a substance which is to be dewatered, such as sludge, is press-dewatered between the filter members 55 by means of a press 56. Since it is possible to press sludge or the like directly by the opposing surfaces of the filter members 55 and since it is inconvenient to load sludge or the like into the area between the filter members 55 and to unload dewatered cake, filter cloths which are used in belt presses are employed as an upper filter cloth 57 and a lower filter cloth 58 which are disposed in opposing relation to each other and in such a manner that they are able to travel intermittently. A substance 60 which is to be dewatered is supplied onto the lower filter cloth 58 from a dewatered substance introducing hopper 59 and then pressed by the filter members 55 through the upper and lower filter cloths 57 and 58, thereby removing water from the substance to be dewatered. However, this apparatus has the disadvantage that, since strong pressing force is applied to thin filter cloths, the filter are severely damaged.
The greatest disadvantage of the above-described conventional dewatering apparatus is that openings (filtrate passages) in the filter cloths are clogged with sludge to resist the passage of filtrate and this makes impossible to effect dehydration, and that, since strong pressure causes sludge to come through the openings in the filter cloths, it is necessary to add a powder having water permeability (e.g., burned ash) to a substance to be dewatered in order to increase the particle size and raise the viscosity. However, addition of such powder increases the amount of substance to be dewatered, such as sludge, and even if the water content of dewatering cake obtained after dehydration is lowered by a large margin, the lowering in the water content is only an observed phenomenon.
For instance, if it is assumed that 20% of a powder is added to sludge as a raw material which is to be dewatered and which has a water content of 80% and the water content after dehydration is 50%, the actual amount of solid content and the actual water content are as follows. In order to facilitate understanding, it is assumed that the amount of a raw material sludge having a water content of 80% is 100 kg/h. In such case, the amount of solid content in the raw material sludge=100 kg/h.times.0.2=20 kg/h; the amount of water content at the time when the water content is 80%=100 kg/h.times.0.8=80 kg/h; and the amount of added powder=100 kg/h.times.0.2=20 kg/h. If the amount of water content after dehydration is represented by W, EQU W=40kg/h
from the following equation: EQU {1-(20kg/h+20kg/h)/(20kg/h+20kg/h+W)}.times.100=50%
Accordingly, the actual water content with respect to only the solid content in the raw material sludge is as follows: EQU {1-(20kg/h)/(20kg/h+40kg/h)}.times.100=67%
In other words, even if the apparent water content is 50%, the actual water content is only about 67%. When the ratio of addition is 10%, the actual water content is found to be 60% by a similar calculation.
Further, since the amount of solid content after dehydration is 40 kg/h and the amount of water content is 40 kg/h, the reduction in volume of the sludge having a water content of 50% is only 80%.
FIG. 19 shows another conventional example wherein a substance 60 to be dewatered which is contained in a tank 61 is subjected to strong press by means of a pressure 56 in a manner similar to that in the apparatus shown in FIG. 18. In the example shown in FIG. 19, fixed filter cloths 62 (or wire nets having a multiplicity of interstices) are employed in place of the movable filter cloths shown in FIG. 18. This example also involves disadvantages similar to those described above and further has the drawback that, since the filter cloths cannot be washed, they cannot endure a long-term service.
In addition, the conventional apparatus wherein a substance to be dewatered is pressed so as to be dewatered by means of a pair of press members has the disadvantage that water which has been removed from the dewatered substance is sucked back into the substance by the effect of a vacuum produced after the substance has been released from pressing force.
One example of the vacuum type dewatering apparatus is disclosed in the specification of Japanese Patent Publication No. 44701/1978, and one example of the dewatering apparatus utilizing the capillary action is disclosed in the specification of Japanese Patent Laid-Open No. 5737/1980, but the dewatering capacity of these conventional apparatuses is limited.
The present invention aims at solving the above-described problems of the prior art and its object is to provide a dewatering method and apparatus therefor wherein a substance which is to be dewatered is pressed by press members each having its press surface formed from a rigid porous material having water absorption and retention properties based on the capillary action to squeeze water from the substance to be dewatered, and the squeezed water is permeated into the rigid porous material to effect dehydration.
Another object of the present invention is to provide a dewatering method and apparatus therefor wherein a sludgy substance which is to be dewatered, such as sludge, is confined in a predetermined space, and while doing so, it is pressed by press members each having its press surface formed from a rigid porous material having water absorption and retention properties based on the capillary action to squeeze water from the substance to be dewatered, and the squeezed water is permeated into the rigid porous material in association with water pressure, thereby effecting dehydration and thus obtaining dewatered cake having a low water content.